Data Watch: Atmospheric CO2 July 2013

I haven’t posted monthly CO2 numbers for a while: although atmospheric CO2 concentration is vitally important for climate outcomes, any one particular month carries little meaning. Nonetheless, I think it worth checking in every quarter or so to look for any nascent deviation from trend.

Key numbers relating to NOAA’s August 5th release of July 2013 mean monthly CO2 concentration are as follows:

July 2013 = 397.23 ppm, +2.93 ppm year-on-year

Twelve Month Average = 394.53 ppm, +2.56 ppm year-on-year

Twelve month average over pre–industrial level = +41.2%

Atmospheric CO2 concentration is the world’s leading risk indicator. Every month, the National Oceanic and Atmospheric Administration (NOAA), a U.S. government federal agency, releases data on the concentration of atmospheric CO2 as measured by the Mauna Loa Observatory in Hawaii. The official NOAA CO2 data source can be found here.

This is the longest continuous monthly measurement of CO2 and dates back to March 1958, when 315.71 parts per million (ppm) of CO2 was recorded. The Intergovernmental Panel on Climate Change (IPCC) uses the year 1750 as the pre-industrialisation reference point, at which date the atmospheric concentration of CO2 was approximately 280 ppm according to ice core measurements.

Atmospheric CO2 displays annual seasonality: concentrations decline from the spring during the growing phase of terrestrial vegetation and rise in the autumn as vegetation dies and decomposes. The cycle is dominated by the northern hemisphere growing season since the northern hemisphere contains over 65% of the globe’s land mass. The cyclical pattern can be seen in the following chart (red line). The black line is the adjustment for seasonality.

The peak for atmospheric CO2 concentration is generally in the month of May, as with this year when the average monthly value reached 399.76 ppm. The newspaper headlines reporting the CO2 concentration breaking above 400 ppm refer to the daily values. The rise in annual CO2 is due to fossil-fuel emissions and land-use change. However, there also exists some minor non-seasonal year-to-year variation related to weather, drought, fire, ocean current changes and volcanic eruptions.

A critical concern is whether the annual average mean increase in CO2 is accelerating. For the decade ending the year 2010, the annual average increase was around 2 ppm. The current 12-month moving average increase is now 2.5 ppm, although there is considerable year-to-year variation as the chart shows.

To achieve the ultimate objective of the Convention to stabilize greenhouse gas concentration in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system, we shall, recognizing the scientific view that the increase in global temperature should be below 2 degrees Celsius, on the basis of equity and in the context of sustainable development, enhance our long-term cooperative action to combat climate change.

The consensus best estimate is that a CO2 concentration of 450 ppm should not be surpassed in order to have a chance of keeping the increase in global mean temperature below 2 degrees Celsius. The current atmospheric CO2 trend suggests that the 450 ppm concentration and, subsequently, 2 degree Celsius threshold will be substantially exceeded.